The invention relates to the field of transmitting digital data by optical means. It is more particularly concerned with transmission at high bit rates on long-haul fiber optic links.
Such transmission uses an optical transmitter connected to an optical receiver by the fiber. The transmitter generally modulates the power of an optical carrier wave from a laser oscillator as a function of the information to be transmitted. NRZ or RZ modulation is very frequently used and entails varying the power of the carrier wave between two levels: a low level corresponding to extinction of the wave and a high level corresponding to a maximum optical power. The variations of level are triggered at times imposed by a clock rate and this defines successive time cells allocated to the binary data to be transmitted. By convention, the low and high levels respectively represent the binary values xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d.
The maximum transmission distance is generally limited by the ability of receivers to detect without error these two power levels after the modulated wave has propagated in the optical link. The usual way to increase this distance is to increase the ratio between the average optical power of the high levels and that of the low levels, this ratio defining the xe2x80x9cextinction ratioxe2x80x9d which is one of the characteristics of the modulation.
For a given distance and a given extinction ratio, the information bit rate is limited by chromatic dispersion generated in the fibers. This dispersion results from the effective index of the fiber depending on the wavelength of the wave transported, and it has the consequence that the width of the transmitted pulses increases as they propagate along the fiber.
This phenomenon is characterized by the dispersion coefficient D of the fiber, which is defined as a function of the propagation constant xcex2 by the equation D=xe2x88x92(2xcfx80c/xcex2)d2xcex2/dxcfx892, where xcex and xcfx89 are respectively the wavelength and the angular frequency of the wave.
The value and sign of the dispersion coefficient D depend on the type of fiber and the transmission wavelength. For example, for the xe2x80x9cstandardxe2x80x9d monomode fibers routinely used, and for xcex=1.55 xcexcm, the coefficient D is positive and has a value of 17 ps/(nm.km). In contrast, the coefficient D is zero for xcex=1.30 xcexcm. The coefficient D can generally be positive, zero or negative depending on the wavelength and the type of fiber used.
If the coefficient D has a non-zero value, to compensate the phenomenon of pulse widening in the case of NRZ or RZ modulation, it has already been proposed to modulate the phase xcfx86 (and therefore the frequency or the angular frequency) of the carrier wave in a manner that correlates to the modulation of the power. The phase xcfx86 corresponds to the convention whereby the electric field of the carrier wave is represented as a function of time t by a complex expression of the type: Ap exp (jxcfx89ot) and the field of a transmitted wave S of amplitude A is represented by: S=A exp [j(xcfx89ot+xcfx86)], where (xcfx89o is the angular frequency of the carrier wave and xcfx86 is the phase of the transmitted wave.
To be more precise, to compensate chromatic dispersion, and if the coefficient D is positive, the phase must decrease on the rising edges of the pulses and increase on their falling edges. The modulated wave is then said to feature a transient negative xe2x80x9cchirpxe2x80x9d. If, in contrast, the coefficient D is negative, the phase modulation must be reversed and the transient xe2x80x9cchirpxe2x80x9d is positive.
A transient xe2x80x9cchirpxe2x80x9d parameter xcex1 is introduced to characterize this modulation, and is defined by the equation xcex1=2P(dxcfx86/dt)/(dP/dt), where P is the power of the modulated wave and xcfx86 is its phase in radians.
For the previously mentioned standard fibers and for values of xcex close to 1.55 xcexcm, for example, the value of the parameter xcex1 must be constant and substantially equal to xe2x88x921 if by approximation xcex1 is regarded as constant.
Another approach proposes to reduce the bandwidth of the signal to be transmitted by appropriate encoding. One particular proposal is to use the xe2x80x9cduobinaryxe2x80x9d code which is well-known in the field of electrical transmission. This code has the property of halving the bandwidth of the signal. According to the standard definition of this code, a signal is used with three levels respectively symbolized by 0, + and xe2x88x92. The binary value 0 is encoded by the level 0 and the value 1 is encoded either by the level + or by the level xe2x88x92 with an encoding rule whereby the levels encoding two successive blocks of xe2x80x9c1xe2x80x9d around a respectively even or odd number of successive xe2x80x9c0xe2x80x9d are respectively identical or different.
Using the duobinary code for optical transmission is mentioned in the article xe2x80x9c10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fiberxe2x80x9d, X.Gu et al., ELECTRONICS LETTERS, Dec. 9, 1993, Vol.29, No.25. According to the above article, the three levels 0, +, xe2x88x92 respectively correspond to three levels of optical power.
French Patent Application No. 94 047 32, publication number FR-A-2 719 175, also describes application of duobinary encoding to the optical field. In the above document, binary xe2x80x9c0xe2x80x9d always corresponds to a low level of the optical power and the symbols + and xe2x88x92 correspond to the same high optical power level and are distinguished by a 180xc2x0 phase-shift of the optical carrier.
The use of that phase inverting duobinary code is also mentioned in the article xe2x80x9cOptical duobinary transmission system with no receiver sensitivity degradationxe2x80x9d, K, Yonenaga et al., ELECTRONICS LETTERS, 16 Feb. 16, 1995, Vol.31, No.4.
In simulations and tests in which the experimental parameters were varied, it was found that an improvement is obtained provided that a phase shift of the carrier wave occurs within each xe2x80x9c0xe2x80x9d which precedes or succeeds each block of xe2x80x9c1xe2x80x9d or each isolated xe2x80x9c1xe2x80x9d. The absolute value of the phase shift can be approximately 180xc2x0. Also, the average optical power of the low levels which encode xe2x80x9c0xe2x80x9d must have a value relative to that of the high levels sufficient to create intersymbol interference favorable to compensating chromatic dispersion. This amounts to saying that the extinction ratio must have a finite value.
The above observations have lead to the definition of a new optical transmission method known as Phase-Shaped Binary Transmission (PSBT). This method is described in European Patent Application EP-A-0 792 036 (Application No. 97400345.1), for example.
The PSBT process requires a transmitter capable of applying an absolute phase shift in the order of 180xc2x0 to the carrier wave within each cell that corresponds to logic xe2x80x9c0xe2x80x9d and which precedes or succeeds any cell containing a logic xe2x80x9c1xe2x80x9d.
A solution using a laser oscillator coupled to an electro-optical power modulator in turn coupled to an electro-optical phase modulator, for example, has the drawback of requiring complex and costly electronic control.
In reality, it is not at all inconvenient for the phase shifts to be effected systematically in each cell containing a logic xe2x80x9c0xe2x80x9d. This leads to a simpler implementation using a xe2x80x9cMach-Zehnderxe2x80x9d interferometer modulator. A modulator of this kind comprises an interferometer structure with an input optical guide that splits into two branches that are combined to form an output guide. Electrodes apply respective electric fields to the two branches. When the input optical guide receives a carrier wave of constant power, two partial waves propagate in the two branches and then interfere at the output. The output guide then supplies a wave whose power and phase depend on the values of the electrical control voltages applied to the electrodes. Phase shifts of approximately 180xc2x0 can be produced at the times when the instantaneous power of the transmitted wave is zero.
To satisfy the conditions for PSBT modulation, the electrical control system must firstly feature amplitude modulation at three main levels as a function of the signal to be sent, in accordance with the duobinary code. It must also feature sustained oscillation at a low amplitude during consecutive sequences of xe2x80x9c0xe2x80x9d. The electrodes must therefore be biased so that in the absence of modulation the DC components of the applied electrical voltages are such that the interference of the two partial waves is as destructive as possible.
If the modulated control signal is applied to only one of the electrodes and the other electrode receives a fixed bias voltage, the optical signal output by the modulator features a non-zero transient xe2x80x9cchirpxe2x80x9d which can be positive or negative, depending on the sequence of binary data encountered and whether the edge is a rising or falling edge.
One solution to the problem of eliminating that uncontrolled transient xe2x80x9cchirpxe2x80x9d is to use xe2x80x9cpush-pullxe2x80x9d control applying a modulated voltage to one of the electrodes, as previously indicated, and a modulated voltage with the opposite phase to the other electrode.
Tests on standard fibers have shown that PSBT modulation achieves transmission distances much greater than those that can be attained with NRZ or RZ modulation. For example, a 10 Gbit/s signal can be transmitted 240 km, although the limit with NRZ modulation is only around 70 km.
However, implementations of PSBT modulators, especially those with an interferometer structure as mentioned above, do not always guarantee optimum transmission quality, regardless of their operating conditions.
For example, long-haul transoceanic transmission optical links include many amplifiers. The noise generated by the amplifiers then seriously degrades the extinction ratio. It is then useful to be able to adjust the extinction ratio at the transmitter end to give it an optimum value, i.e. a value high enough to allow for the amplifiers but low enough for intersymbol interference to compensate widening of the pulses due in particular to chromatic dispersion. An adjustment of this kind is difficult to implement in the control function of PSBT modulators, however.
Studies have shown that with PSBT modulation transmission distances can be increased by introducing a transient xe2x80x9cchirpxe2x80x9d whose sign and optimum value depend in particular on the dispersion coefficient D of the fiber, on the required transmission distance, and on non-linear effects (Kerr effect). As previously mentioned, the interferometer modulator solution cannot readily impose a transient xe2x80x9cchirpxe2x80x9d of given sign and value.
Also, the aim of the invention is to propose a transmission system which is more flexible to use and which is easier to optimize for each type of optical link and for each transmission distance.
To be more precise, the invention consists in a system for transmitting an optical signal in the form of an optical carrier wave modulated as a function of an input binary electrical signal, a clock rate defining successive time cells delimiting in the input signal first or second modulation levels, said device including a first electro-optical modulator adapted to respond to said input electrical signal by supplying a controlled phase optical signal having optical power modulated between low levels and high levels respectively corresponding to said first and second modulation levels of the input signal, and a phase shift within each time cell that contains a low power level and which precedes or succeeds a cell that contains a high power level, said system including a second electro-optical modulator controlled by said input signal and optically coupled to the first electro-optical modulator to apply complementary power and/or phase modulation to said controlled phase optical signal so as respectively to modify its extinction ratio and/or to apply a transient xe2x80x9cchirpxe2x80x9d to it.
To facilitate setting the modulation characteristics of the system, the first electro-optical modulator preferably does not apply any transient xe2x80x9cchirpxe2x80x9d to the controlled phase optical signal.
This latter condition can be met by using a xe2x80x9cMach-Zehnderxe2x80x9d interferometer structure with xe2x80x9cpush-pullxe2x80x9d control.
To be more precise, in this embodiment of the invention, the first electro-optical modulator includes:
a xe2x80x9cMach-Zehnderxe2x80x9d interferometer structure wherein an input optical guide splits into two branches to guide two partial waves, said two branches combining again to form an output guide, respective electrodes being provided to apply electrical fields to said two branches, and
a control circuit for applying to the electrodes respective control voltages having DC components between modulation components in phase opposition, said DC components being such that in the absence of modulation components said partial waves interfere destructively.